1. Field of the Invention
The present invention relates to a thin film device and more particularly to a thin film device wherein an oxide high temperature superconductor is employed and which is useful for example in making a lamination-type Josephson device.
2. Discussion of the Background
In recent years, a research has been actively conducted on an oxide superconductor containing Cu--O layers in the crystal structure, as a superconductor which may be substituted for conventional superconductors of the Nb or A15 type compound (.beta.-W type crystal structure) such as Nb.sub.3 Sn or Nb.sub.3 Ge.
As a typical material for such an oxide superconductor, a Y--Ba--Cu--O (YBCO) type (critical temperature Tc&gt;90K), a Bi--Sr--Ca--Cu type (critical temperature Tc=80-110K), and a Tl--Ba--Ca--Cu--O type (critical temperature Tc=90-120K) are known.
The most important technique in a case where such a material is used as a Josephson device as an electronic device, is formation of a Josephson junction by a vapor deposition method, a sputter vapor deposition method or laser ablasion.
However, it is known that in the above mentioned material, the coherent length is very short. It was thought that for a good Josephson Junction, the size of the disorder at the interface-should be less than the coherent length. The surface tends to readily deteriorate. Therefore, when a Josephson junction is formed by laminating thin films of such a material with an interlayer interposed therebetween to form a space, the lattice matching between the superconductor material and the interlayer tends to be poor, and there are drawbacks such that the properties of the adjacent superconductor layers tend to deteriorate or a leakage current from the interlayer is likely to be observed. It is difficult to obtain a S/N/S (superconductor/normal conductor/superconductor) junction with good reproducibility, or even in the case of a S/I/S (superconductor/insulator/superconductor) junction, a leakage current is likely to result, or a superconducting connection tends to be hardly obtained.
To solve such drawbacks, the following methods (a) to (c) have been proposed, for example, in the case of a laminated thin film device.
(a) A method wherein an insulating film is formed on an oxide superconductor, and then a conventional type (phonon type) superconductor (such as Nb or a Nb type alloy, or Pb or a Pb compound) is laminated thereon. PA1 (b) A method wherein a metal superconductor material having a relatively small amount of oxygen or a material commonly used for a substrate such as MgO, SrTiO.sub.3 or ZrO.sub.2, is laminated as an interlayer on an oxide superconductor, and a superconductor is laminated thereon. PA1 (c) A method wherein on an oxide superconductor, an insulator layer is formed by substituting the rare earth element or alkaline earth metal of such an oxide superconductor by other atoms, and an oxide superconductor is laminated on such an insulator layer.
However, in the case of the above method (a), a conventional superconductor is employed, whereby there is a problem that the thin film device can be operated only at a temperature corresponding to the temperature of liquefied helium.
Method (b) has a problem that in the case where a metal superconductor material having a relatively small amount of oxygen is used as the interlayer, it is difficult to obtain a sharp interface since the controllability of the diffusion of oxygen during annealing tends to be poor, or in the case where a substrate material like MgO is employed, it tends to be difficult to obtain a film having good properties because of the diffusion of Mg or the like.
Method (c) has a problem that since the bond between the superconducting Cu--O layer and the rare earth element or the like is weak, and the above mentioned substitution must be conducted in a large amount (at least 50%) in order to convert the superconducting properties to the insulating properties.
The present inventors have conducted extensive research with an aim to solve the above problems and have found a method whereby a S/N/S or S/I/S structure can be prepared without substantially changing the structure of the oxide superconductor.
Namely, the present invention has been accomplished by paying a particular attention to the specificity of the basic structure of an oxide superconductor containing Cu, particularly to the two dimensional characteristic thereof.